Figure out the list of files to include in the source
distribution, and put it in 'self.filelist'. This might involve
reading the manifest template (and writing the manifest), or just
reading the manifest, or just using the default file set -- it all
depends on the user's options and the state of the filesystem.

Create the source distribution(s). First, we create the release
tree with 'make_release_tree()'; then, we create all required
archive files (according to 'self.formats') from the release tree.
Finally, we clean up by blowing away the release tree (unless
'self.keep_temp' is true). The list of archive files created is
stored so it can be retrieved later by 'get_archive_files()'.

Create the directory tree that will become the source
distribution archive. All directories implied by the filenames in
'files' are created under 'base_dir', and then we hard link or copy
(if hard linking is unavailable) those files into place.
Essentially, this duplicates the developer's source tree, but in a
directory named after the distribution, containing only the files
to be distributed.

Prune off branches that might slip into the file list as created
by 'read_template()', but really don't belong there:
* the build tree (typically "build")
* the release tree itself (only an issue if we ran "sdist"
previously with --keep-temp, or it aborted)
* any RCS or CVS directories